It is known in the art to use various optical modules that are assembled into a desired configuration to perform a single specified optical function. Such configuration may take the form of a scientific instrument, or may find employment in a commercial spectroscopy application. It is usually advantageous to make each module as compact as possible.
A spectroscope, an instrument which produces a spectrum from input light, is one particularly useful example of such an optical instrument. The term “spectrum” is meant to encompass less than a complete range of wavelengths, for example, the collection of wavelengths emitted by a sample under excitation or passed by a sample filtering input light energy. Another example is a spectrograph which is a spectroscope provided with a recording device, or other light-capture means, or the like to receive and record or otherwise process the spectrum generated. The present invention relates to an optical arrangement particularly useful in spectrographs, and that term will be used hereinafter. However, it will be understood that the novel optics described herein can be employed in spectroscopes or spectrometers and generally all applications where recording, quantification or similar capabilities are required and the invention extends to such novel spectroscopic and spectrometric applications employing the inventive optics. To the extent that the invention may be applied to output a single spectral band or to provide a scanned output comprising a series of individual spectral bands, the term “spectrometer” should also be understood to include monochromators.
Diffraction grating spectrographs use one or more diffraction gratings to diffract input light into a spectrum of specific wavelengths or spectral bands. In a typical configuration, spectrographs are designed to select a single wavelength, or a narrow spectral band from the input light, for examination or recordal.
In one known embodiment of spectrometer employing a planar diffraction grating, a concave mirror is illuminated by a point source whose spectrographic composition is to be analyzed. The light from the point source is collimated by the concave mirror to form a parallel bundle of rays, which are caused to fall upon the surface of a planar diffraction grating. This concave mirror is known as a collimator in a typical spectrometer instrument. Because the planar diffraction grating has a number of grooves etched in its surface, light falling on the surface of the diffraction grating is diffracted, that is, reflected at an angle which is a function of the wavelength of the light. If the input light source comprises a number of wavelengths, the result is that light of different wavelengths will be diffracted, or reflected, at an angle which is a function of wavelength.
The diffracted light may then be received by a second concave mirror which focuses the diffracted light to form an image of the point source under analysis. However, because light of different wavelengths has been diffracted at different angles, the point source is imaged by the second concave mirror, also known as a focusing mirror, at different points for different wavelengths. Accordingly, it is possible to select out individual wavelengths, or more precisely a narrow region of the spectrum, or spectral band, consisting essentially of a single wavelength, to measure the intensity of the same and to utilize this information, for example for elemental analysis of an emissive source material.
Spectrometric elemental analysis of samples has many industrial uses. For example, in the case of the analysis of industrial slag, such as might be obtained from a crucible filled with molten metal in a steel furnace, the slag may be exited into a plasma, and the emission spectrum analyzed and measured with a spectrometer. The wavelengths appearing in the plasma emission spectra indicate the nature and quantity of the impurities in the slag, enabling plant operators to adjust production parameters to achieve a desired product.
While the above discussion has centered on spectrometer devices using mirrors, and such devices are usually preferred because of the quality of imaging using mirrors, it is possible to construct devices using focusing lenses, such as convex lenses or compound multi-element lenses having an overall convex optical characteristic. In principle, it is also possible to combine lenses and mirrors in an instrument.
It is also noted that diffraction gratings in spectrometers may be either classical mechanically ruled diffraction gratings of the type invented and made by applicant's assignee at the beginning of the 1800's, or holographic diffraction gratings of the type pioneered by applicant's assignee since the 1960's.
It is also known that spectrometers may be constructed using concave diffraction gratings, such as concave holographic diffraction gratings of the type invented by Flamand in the late 1960's working at the applicant company as illustrated by his U.S. Pat. No. 3,628,849.
A Littrow-mounted system is a relatively common method of utilizing large plane reflection gratings, providing simplicity and good optical quality arising from the use of a single mirror to perform both collimating and focusing functions. Moreover, in this configuration, the collimating and focusing functions are both performed in the same geometric space, resulting in efficient use of that space. In a typical Littrow setup, a mirror delivers parallel incident light from an input point source to the grating, and focuses diffracted light received from the grating to an output point often proximate the input point source. In such devices, a single mirror acts as both collimator and focusing element at once, minimizing the number of optical elements required.
In addition to its simplicity, employing the Littrow configuration is particularly desirable for its high quality output. Because the input and output light beams traverse the same optical path, in opposite directions, optical aberrations in the collimating and focusing components are auto-corrected, or self compensating, so that image quality is diffraction limited, i.e. limited by the physical properties of the optical system not by the deficiencies of the optics.
A particular drawback of such conventional Littrow-mounted grating configurations is the difficulty and expense of providing a grating with a central optical opening. Another drawback is that undue stray light may be returned to the aperture by the mirror. It would be desirable to provide a spectrometer or comparable optical system, which did not suffer from these drawbacks.